Power conversion systems convert electrical power from one form to another and may be employed in a variety of applications such as motor drives for powering an electric motor using power from an input source. Such power converters have been extensively employed in medium voltage motor drives and other applications in which electrical power needs to be converted from DC to AC or vice versa. Typically, the power converter is constructed using electrical switches actuated in a controlled fashion to selectively convert input power to output power of a desired form such as single or multi-phase AC of a controlled amplitude, frequency and phase to drive an AC motor according to a desired speed and/or torque profile, often in the presence of varying load conditions. Such conversion apparatus is commonly constructed as an inverter for converting DC to AC and/or a rectifier if the conversion is from AC to DC power, where the input and/or output AC power connections are often a multi-phase. Multi-phase converters are often constructed using an array of high-voltage, high-speed semiconductor-based switching devices which are selectively actuated through pulse width modulation (PWM) to couple the AC connections with one or the other of the DC bus terminals, with the timing of the array switching determining the power conversion performance. In motor drive applications, for example, the timed control of inverter switch activations is used to provide variable frequency, variable amplitude multi-phase AC output power from a DC bus to control a driven motor across wide voltage and speed ranges to control the motor speed and/or torque in the presence of varying load conditions.
Current source converters (CSC) are widely used in high power medium voltage (e.g., 2.3-13.8 kV) applications, which generally use a device switching frequency of several hundred hertz or less to mitigate device switching loss and maintain rated device thermal operating conditions. For these converters, several different modulation schemes can be employed, including selective harmonic elimination (SHE), trapezoidal pulse-width modulation (TPWM), and space vector modulation (SVM). Among these, SHE is effective for reducing low order harmonic distortion at low switching frequency. However, the modulation index of SHE is usually fixed due to implementation difficulties, and thus the SHE modulation approach typically does not allow control flexibility. Conventional SVM and TPWM modulation techniques allow modulation index adjustment, but generally suffer from high levels of low order harmonic distortion, particularly the 5th and 7th harmonics that are often close to the resonance frequency of motor drive AC filters. Accordingly, there is a need for improved power conversion systems and switching device modulation techniques by which low order harmonic distortion can be controlled while allowing modulation index control in conversion of electrical power for motor drives or other power conversion systems.